1. Field of the Invention
This invention relates to a process of preparing green peas, green beans, spinach and other like green vegetables for canning or freezing and is particularly related to a method of preparing green vegetables to preserve the color of the canned vegetable during processing and storage.
2. The Prior Art
In the conventional method of canning green vegetables such as green beans, peas and spinach, the vegetable after being washed, is subjected to a blanching operation of 3 to 5 minutes duration. The primary purpose of the blanch is to inactivate enzymes and expel air which would otherwise result in off-flavors.
After the blanching operation, the green vegetables are filled in cans, covered with a brine solution consisting of sugar and/or salt, and the cans are thereafter sealed and sterilized.
This method of canning of vegetables has the disadvantage that the canned product does not retain the fresh green color of the growing vegetable, but assumes a dull yellow-olive color which is unpleasant to the eye and, consequently, is not as appetizing as it might be if the fresh green color were retained. The undesirable change in color appears to be due to hydrolytic and oxidative changes which accompany the sterilization of commercially canned green vegetables. In many cases, these degradative changes in canned vegetables are accelerated by the production of organic acids during thermal processing. Thus, the normal pH value for the fluids expressed from raw peas is about 6.6 to 7.0 whereas the product after sterilization ranges from 5.8 to 6.4. This decrease in pH constitutes an increase in acidity which causes the destruction of the natural green pigment during thermal processing and subsequent storage, i.e., the degradation of green colored chlorophyll to yellow-olive colored pheophytin.
Prior art processes for the retention of green color in canned green vegetables generally require an additive compound to achieve the color retention (i.e., to prevent the conversion of chlorophyll to yellow-olive colored pheophytin). These additives can be classified into two principal groups (1) alkalizing or alkalizing-buffering agents such as the hydroxides and carbonates of alkali and alkaline earth metals such as sodium, calcium and magnesium and (2) metallic salts such as the chlorides and acetates of zinc and copper.
For example, U.S. Pat. Nos. 1,908,795, 2,186,003, 2,189,774, 2,305,643 and 2,318,426 teach preserving the color of green vegetables by maintaining the canned product at an alkaline pH by incorporating alkaline reagents such as calcium hydroxide, magnesium hydroxide, magnesium oxide, magnesium carbonate in the blanch water and/or brine in which the vegetables are processed. U.S. Pat. No. 2,875,071 teaches incorporating a water soluble alkaline compound in the lining of cans in which green vegetables are packed whereby the alkaline compound, such as magnesium hydroxide, is slowly released into the brine to maintain the pH of the canned product at an alkaline level e.g., 8.0, and thereby preserve the green color.
In canning processes in which the color retention of the green vegetable is not dependent upon maintaining the pH at an alkaline level, a metal salt is added to the vegetable brine to provide metal ions to replace the magnesium ion lost from the chlorophyll during thermal processing of the green vegetable. For example, in an article entitled "Spectrophotometric Study of the Green Color in Okra," Association of Official Agricultural Chemists 26, 134-139 (1943) the authors report that the natural green color of okra is preserved during canning by the incorporation of a small amount of zinc chloride in the vegetable brine prior to sealing and thermal processing. The authors conclude that color stabilization is due to zinc ion replacing the magnesium ion lost from chlorophyll molecule during processing to effect the synthesis of a stable green compound.
In a second article entitled "Color Reversion in Processed Vegetables 1. Studies in Regreened Pea Purees" Journal of Food Science, 30, 312 (1965) the authors report the results of an investigation of a regreening phenomenon sometimes observed in the commercial vegetable packing industry. The authors had observed that green pea puree aseptically packed in glass jars and stored at 32.degree. C. (90.degree. F.), exhibited, after 6 months, a reversion to a green color which closely resembled the color of unprocessed peas. Analysis of pigment isolated from the puree indicated the formation of copper and zinc complexes. The authors then formed zinc and copper complexes by refluxing, for 20-60 minutes, pea puree to which zinc and copper compounds had been added at levels of 5 to 100 parts per million (ppm).
Although the art discussed above discloses a variety of processes for the retention of natural color in green vegetables, none of these processes are in current commercial use. A recent patent, U.S. Pat. No. 4,104,410, discloses that the drawback in the use of alkalizing agents for the retention of color in canned green vegetables is that the intensity of the retained green color decreases gradually when the canned green vegetable is kept in storage at room temperature. After about a 1 year storage period, due to the high pH level, an ammoniacal flavor (barn-yard flavor) develops. The patentee, who is also the patentee of U.S. Pat. No. 2,875,071, further discloses that brine additives such as salts and coordination compounds of zinc and copper are included in compounds which are toxic and not allowed in most countries as food additives. The process claimed in U.S. Pat. No. 4,104,410 achieves green color retention in vegetables without the addition of chemicals wherein the conventional blanching step is eliminated from the canning process and the vegetable is maintained in an aerobic environment at a temperature below its thermal injury temperature, e.g., 120.degree.-140.degree. F. up to the time of sterilization and thereafter sterilized in a high (275.degree.-425.degree. F.) temperature/short time (HTST) process. The process disclosed in U.S. Pat. No. 4,104,410 is also not in commercial use.
There is a need in the art for a canning process for green vegetables wherein color retention is achieved without the drawbacks mentioned in U.S. Pat. No. 4,104,410. For example, consumer research studies have shown a direct correlation between the color of green vegetables and the consumers perceived taste satisfaction. Many different stimuli contribute to the taste satisfaction of green vegetables and, according to the research, the foremost among these is natural fresh color. The only method known today to preserve the natural color of green vegetables is freezing. Frozen vegetables are costly to process and store and, because of rising energy costs, will be even more costly in the future.
Recent research (1981) by the National Food Processors Association has compared the effect of cooking on the nutrient content of vegetables such as peas and spinach in raw, frozen and canned form. The results showed that after cooking there was no real difference between canned and frozen vegetables and only small nutritionally insignificant differences existed between canned and fresh vegetables in most of the 23 nutrients tested.